This application is related to and claims priority, under 35 U.S.C. xc2xa7 119, from Japanese Patent Application No. 2000-104492, filed on Apr. 6, 2000, and Japanese Patent Application No. 2001-056186 filed on Mar. 1, 2001, and the entire contents of both Japanese patent applications are hereby incorporated by reference herein.
1. Field of the Invention
The present invention relates to a sheet feeding device having a base plate which is configured to be rotatable so that sheets, which are stacked on the base plate, are pressed against a feeding roller in order to be fed, and an image forming apparatus using the sheet feeding device.
2. Discussion of Background
Japanese Patent Laid-Open Publication No. 5-170347 discloses a sheet feeding device which feeds sheets stacked on a rotatable base plate with a feeding roller. Beginning with the uppermost sheet of the stacked sheets, the sheets are fed one by one by pressing the base plate toward the feeding roller so that the stack of sheets stacked on the base plate are pressed against and into contact with the feeding roller. As the number of sheets stacked on the base plate decreases, the base plate rotates toward the feeding roller such that an uppermost sheet, of the stack of sheets stacked on the base plate, is pressed against and into contact with the feeding roller.
It is known to provide a friction member on an upper surface of the base plate in the vicinity of a part of the base plate opposed to the feeding roller, i.e., at a part of a free end side of the base plate. This gives a frictional resistance to a bottom sheet of the stack of sheets stacked on the base plate. Thus, the bottom sheet, of the stack of sheets stacked on the base plate, is prevented from being pulled out from its position against the base plate when the uppermost sheet is fed.
The above-described sheet feeding device must be configured so that only the uppermost sheet is reliably pulled out from the stack of sheets stacked on the base plate. In other words, the uppermost sheet of the stack of sheets stacked on the base plate must slip relative to adjacent and subsequent sheets stacked on the base plate. Further, even when the number of sheets in the stack of sheets stacked on the base plate has been decreased such that the bottom sheet of the stack of sheets receives a gripping force from the feeding roller, the bottom sheet must be prevented from being pulled out from its position against the base plate via the frictional resistance of the friction member against the bottom sheet. Furthermore, the sheet feeding device must be configured such that when only one sheet, i.e., the last sheet of the stack of sheets, remains on the base plate, the feeding roller pulls out the last sheet using a pulling force which is larger than the frictional resistance which the friction member provides with respect to the last sheet of the stack of sheets stacked on the base plate.
The sheet feeding device includes a feeding roller which feeds out the uppermost sheet of the stack of sheets stacked on the base plate via a pulling force Fa of the feeding roller acting on the uppermost sheet. A frictional resistance Fb acts between each sheet of the stack of sheets stacked on the base plate. A friction member is located at a free end side of the base plate and provides a frictional resistance Fc to a bottom sheet of the stack of sheets stacked on the base plate. The sheet feeding device is configured so that the relationship Fa greater than Fc greater than Fb is satisfied.
However, even when a sheet feeding device is designed so as to satisfy the relationship Fa greater than Fc greater than Fb, sheets may still be double-fed (i.e., a plurality of sheets are pulled out at the same time from the stack of sheets stacked on the base plate). Of course, the occurrence of the double-feeding phenomenon in part depends upon the quality of the sheets, the environmental conditions (e.g., temperature and humidity), the state of static electricity, and the resilience of the sheets, etc. Therefore, for preventing such a double feeding of sheets, a separation device is generally arranged in the vicinity of an end of the base plate at the downstream side of the sheet feeding device in the sheet feeding direction so as to oppose the feeding roller. The relationship Fa greater than Fc greater than Fb is further desired to be satisfied such that the burden on the separation device is decreased and thereby the sheet separation performance is enhanced and each sheet of the stack of sheets stacked on the base plate are reliably fed from first sheet to last sheet.
Changes in the quality of the sheets, the environmental conditions, such as temperature and humidity, the state of the static electricity, and the resilience of the sheets affect the pulling force Fa, and the frictional resistances Fb and Fc.
Moreover, as the number of sheets of the stack of sheets stacked on the base plate increases, the base plate is positioned lower relative to the feeding roller so as to contract a base plate pressing member which upwardly biases the base plate. Thereby, the pulling force Fa, acting on the uppermost sheet of the stack of sheets stacked on the base plate, increases, but only slightly.
The frictional resistance Fc of the friction member provided with respect to the bottom sheet of the stack of sheets stacked on the base plate changes according to the variation in the friction coefficient of the friction member. Moreover, because a reaction force, corresponding to the pulling force Fa from the feeding roller, acts on the friction member via the stack of sheets stacked on the base plate, the frictional resistance Fc further changes. Thus, the frictional resistance Fc changes goes through a larger magnitude of change than does either the pulling force Fa or the frictional resistance Fb.
When the sheet feeding device is configured such that the base plate is moved toward the feeding roller for sheet feeding and is moved away from the feeding roller during the intervals of sheet feeding, as disclosed in Japanese Patent Laid-Open Publication No. 9-202475, the position of the base plate, when moved toward the feeding roller, changes according to the amount of play of a rotation support part of the base plate. This further increases the variation in the frictional resistance Fc.
Further, when the sheet feeding device is configured such that the base plate is inclined downwardly (i.e., from the side of the rotation center of the base plate toward the side on which the feeding roller is located), in order to reduce the installation space for the sheet feeding device in the horizontal direction, the inclination of the base plate relative to the vertical direction increases as the number of sheets of the stack of sheets stacked on the base plate increases. Thus, the stack of sheets stacked on the base plate tends to plunge between the feeding roller and the friction member under the weight of the stack of sheets. Such a plunging of the stack of sheets is more remarkable as the size of the sheets increases and thus, the phenomenon of double feeding of the sheets tends to occur more easily. Therefore, to avoid double feeding of sheets, the value of the frictional resistance Fc needs to be set relatively high. It is difficult to properly set the frictional resistance Fc since it should correspond to the inclining angle of the base plate.
The present invention has been made to solve the above-discussed and other problems of the prior art.
Preferred embodiments of the present invention provide a sheet feeding device that can reliably separate and feed a stack of sheets stacked on a base plate from a first sheet to a last sheet of the stack of sheets.
According to a preferred embodiment of the present invention, a sheet feeding device includes a feeding roller, which is driven to rotate, and a base plate on which to stack sheets to form a stack of sheets. The sheet feeding device is preferably configured to rotate around a supporting axis thereof in directions to contact and separate from the feeding roller. A base plate pressing member is configured to press the base plate toward the feeding roller. A first friction device is arranged in a vicinity of a first part of an upper surface of the base plate opposing the feeding roller. The first friction device provides the bottom sheet of the stack of sheets stacked on the base plate with a first frictional resistance greater than a predetermined value. A second friction device is arranged upstream of the first friction device in a sheet feeding direction and provides the bottom sheet of the stack of sheets stacked on the base plate with a second frictional resistance greater than the predetermined value discussed with respect to the first frictional resistance.
Thus, when the base plate is biased upwardly by the base plate pressing device such that an uppermost sheet of the stack of sheets stacked on the base plate contacts the feeding roller, the first friction device provides a first frictional resistance to a bottom sheet of the stack of sheets stacked on the base plate which is greater than the predetermined value, while receiving a reaction force from the feeding roller via the stack of sheets stacked on the base plate. The second friction device provides a second frictional resistance to the bottom sheet of the stack of sheets stacked on the base plate which is greater than the predetermined value discussed with respect to the first frictional resistance. Thus, the frictional resistance of the last sheet of the stack of sheets stacked on the base plate is set equal to a sum of the first frictional resistance of the first friction device and the second frictional resistance of the second friction device. Accordingly, the frictional resistance of the first friction device, which is opposed by a reaction force from the feeding roller varying according to the number of sheets of the stack of sheets stacked on the base plate, can be set relatively low. Thus, the influence of the reaction force from the feeding roller is decreased, and thereby the frictional resistance on the bottom sheet of the stack of sheets stacked on the base plate can be easily set to a desired value with the second friction device, which receives a relatively small pressing force from the base plate pressing device because of its position near the supporting axis and which most likely does not receive the influence of the reaction force from the feeding roller. Accordingly, the variation in the frictional resistance provided on the bottom sheet of the stack of sheets stacked on the base plate can be easily suppressed, so that the stack of sheets stacked on the base plate can be reliably separated one by one so as to prevent double-feeding.
In the above-described sheet feeding device, the second friction device may include a second friction member arranged on a second part of the upper surface of the base plate, upstream of the first friction device in the sheet feeding direction, and configured to provided the bottom sheet with the second frictional resistance. Accordingly, by setting material and surface coarseness of the second friction member, the second friction device can be appropriately configured.
Further, the second friction device may include an elastic sheet thrusting device. The elastic thrusting device is arranged upstream of the first friction device in the sheet feeding device and is configured to thrust the stack of sheets stacked on the base plate to a second part of the upper surface of the base plate so that the bottom sheet is provided with the second frictional resistance via the second part of the upper surface of the base plate. In this case, the sheet thrusting device can be appropriately configured by setting the coefficient of elasticity and the dimension thereof.
Furthermore, the second friction device may include a second friction member. The second friction member is arranged on a second part of the upper surface of the base plate, upstream of the first friction device in the sheet feeding direction. The second friction device may also include an elastic sheet thrusting device. The elastic sheet thrusting device is arranged upstream of the first friction device in the sheet feeding direction and is configured to thrust the stack of sheets stacked on the base plate to the second friction member. The second friction member is configured to provide the bottom sheet, of the stack of sheets stacked on the base plate and that are thrust by the sheet thrusting device, with the second frictional resistance. In this case, the bottom sheet of the stack of sheets stacked on the base plate is provided with the second frictional resistance by thrusting of the stack of sheets to the second friction member with the sheet thrusting device, and thereby the bottom sheet of the stack of sheets stacked on the base plate is provided with the second frictional resistance via synergism of the thrusting force of the sheet thrusting device and friction between the second friction member and the bottom sheet. Therefore, relatively large freedom is obtained in a design of the sheet thrusting device and the second friction member.
Still furthermore, the sheet feeding device may include a base plate moving device configured to allow the base plate to move toward the feeding roller when the feeding roller feeds the uppermost sheet of the stack of sheets stacked on the base plate, such that the uppermost sheet of the stack of sheets contacts the feeding roller so as to be fed by the feeding roller, and to cause the base plate to retreat from the feeding roller when the fed sheet is conveyed to a conveying member in a downstream side of the sheet feeding device.
Accordingly, even when a structure in which a feeding roller is continuously rotated is employed in the sheet feeding device, the stack of sheets stacked on the base plate can be intermittently fed by movement of the base plate toward the feeding roller and by the retreating movement of the base plate from the feeding roller. In this case, because the sheet feeding device includes the base plate moving device, the position of the base plate when moved toward the feeding roller may change more greatly than when a structure not having the base plate moving device is employed in the feeding device. Therefore, the first frictional resistance, provided on the bottom sheet of the stack of sheets stacked on the base plate by the first friction member, may also change more greatly than when the structure not having the base plate moving device is employed, according to a change in the reaction force from the feeding roller. However, by setting the first frictional resistance, provided on the bottom sheet of the stack of sheets stacked on the base plate by the first friction member, to be relatively small and by setting the second frictional resistance, provided on the bottom sheet of the stack of sheets stacked on the base plate via the second friction member which most likely does not receive the influence of the reaction force from the feeding roller, to a desired value, even when the structure in which the base plate is moved toward and retreated from the feeding roller is used, the variation in the frictional resistance, provided on the bottom sheet of the stack of sheets stacked on the base plate, can be easily suppressed.
Further, in the above sheet feeding device, the base plate may be downwardly inclined from the supporting axis side toward the feeding roller side thereof.
When a structure in which the sheet stacking surface of the base plate is inclined downwardly from the side of the supporting axis toward the side of the feeding roller is employed in the sheet feeding device, for example, for reducing the installation space for the sheet feeding device in the horizontal direction, as the number of stacked sheets on the base plate increases, the inclination angle of the base plate increases relative to the vertical angle, and thereby the stack of sheets stacked on the base plate tend to plunge between the feeding roller and the second friction member under the force of their own weight. However, by appropriately setting the first and second frictional resistances, to be provided to the bottom sheet of the stack of sheets stacked on the base plate via the first and second friction devices, respectively, as described above, such plunging of the stack of sheets is prevented.
Furthermore, when a pulling force of the feeding roller acting on the uppermost sheet of the stack of sheets stacked on the base plate is Fa, a frictional resistance acting between the stack of sheets stacked on the base plate is Fb, the first frictional resistance provided on the bottom sheet of the stack of sheets stacked on the base plate via the first friction device is Fc, the second frictional resistance provided on the bottom sheet of the stack of sheets stacked on the base plate via the second friction device is Fd, and a third frictional resistance provided on the bottom sheet of the stack of sheets stacked on the base plate via a part of the upper surface of the base plate, where the first or second frictional resistance Fc or Fd of the first or second friction device, respectively, is not provided on the bottom sheet, is Fe, a relationship of Fa greater than (Fc+Fd+Fe) greater than Fb may be satisfied. In addition, a relationship of Fc greater than Fd may be satisfied.
Accordingly, when the uppermost sheet of the stack of sheets stacked on the base plate is to be fed, the uppermost sheet easily slips relative to adjacent and subsequent sheets of the stack of sheets stacked on the base plate. Further, even when a sheet on the base plate is the last sheet of the stack of sheets, the last sheet can be pulled out from its position against the base plate while resisting the frictional resistances provided by the first and second friction devices. Thus, the reliability of separating and feeding the stack of sheets stacked on the base plate, from a first sheet to a last sheet of the stack of sheets, can be further enhanced. Also, the sheet separation performance of the feeding roller is enhanced, and thereby the burden on a separation/friction member, arranged on the downstream side of the sheet feeding roller in the sheet feeding direction, can be reduced, so that the reliability of separating the stacked sheet is further enhanced.
Further, the sheet feeding device may include a separation/friction member arranged to oppose the feeding roller and configured to prevent other sheets, besides the uppermost sheet of the stack of sheets stacked on the base plate, from being fed by the feeding roller. Accordingly, even when the feeding roller pulls out a plurality of sheets from the stack of sheets stacked on the base plate, double feeding of the sheets is prevented by the frictional resistance of the separation/friction member.
Furthermore, in the above sheet feeding device, the sheet thrusting device may include a sheet member having elasticity. Thereby, the structure of the sheet thrusting device can be simplified, so that the sheet feeding device can be made inexpensive.
Further, at least one of the first friction device and the second friction member may be formed from cork or resin, so as to be inexpensive.
According to another embodiment of the present invention, an image forming apparatus includes a sheet feeding device having a feeding roller that is driven to rotate and a base plate to stack sheets thereupon. The sheet feeding device is configured to rotate around a supporting axis thereof in directions to contact and separate from the feeding roller. In the sheet feeding device, a base plate pressing member is configured to press the base plate to be rotated toward the feeding roller such that an uppermost sheet of the stack of sheets stacked on the base plate contact the feeding roller so as to be fed by the feeding roller. The sheet feeding device includes a first friction device, which is arranged on an upper surface of the base plate in a vicinity of a first part of the upper surface of the base plate opposing the feeding roller to provide a bottom sheet of the stack of sheets stacked on the base plate with a first frictional resistance which is greater than a predetermined value. The sheet feeding device also includes a second friction device, which is arranged upstream of the first friction device in a sheet feeding direction to provide the bottom sheet of the stack of sheets stacked on the base plate with a second frictional resistance greater than the predetermined value discussed with respect to the first friction device. The image forming apparatus further includes a sheet conveying member configured to receive and convey the sheet fed from the sheet feeding device, and a printer configured to print an image on the sheet conveyed by the sheet conveying member.